Both fit and fitted are standard as past tense and past participle of fit1: The new door fit (or fitted) the old frame perfectly. The suit had fitted (or fit) well last year.Fitted is somewhat more common than fit in the sense “to adjust, make conform”: The tailor fitted the suit with a minimum of fuss. In the passive voice, fitted is the more common past participle: The door was fitted with a new handle.

fit

"be suitable," probably from early 15c.; "to be the right shape," 1580s, from fit (adj.). Related: Fitted; fitting. Fitted sheets is attested from 1963.

fit

n.3

part of a poem, Old English fitt, of unknown origin.

fit

n.1

1823, "the fitting of one thing to another," later (1831) "the way something fits." Originally "an adversary of equal power" (mid-13c.), obscure, possibly from Old English fitt "a conflict, a struggle" (see fit (n.2)).

fit1 (fĭt)v.fit·ted or fit, fit·ted, fit·ting, fits To be the proper size and shape. adj.fit·ter, fit·test Physically sound; healthy. n. The degree of precision with which surfaces are adjusted or adapted to each other in a machine, device, or collection of parts.